1. Field of the Invention
The present invention generally relates to the field of high frequency waveguide devices, and more specifically to a cavity resonator incorporating a waveguide filter which may be advantageously employed in a free-electron laser.
2. Description of the Related Art
In a free-electron laser (FEL) such as disclosed in U.S. Pat. No. 4,438,513, entitled "SHORT WAVELENGTH FREE ELECTRON LASER USING LOW ENERGY ELECTRONS", issued Mar. 20, 1984 to L. Elias et al, an electron beam interacts with a magnetic "wiggler" field to produce coherent radiation at microwave or optical frequencies. The interaction region is enclosed in a cavity resonator which includes a hollow waveguide member provided with reflectors at its opposite ends to create resonance at the frequency of the microwave or optical radiation, and thereby enhancing gain and stimulated emission.
The reflectors may be optical mirrors or microwave reflectors, or blazed corrugations or vanes which satisfy the Bragg conditions for gratings such as disclosed in U.S. Pat. No. 4,697,272, entitled "CORRUGATED REFLECTOR APPARATUS AND METHOD FOR FREE ELECTRON LASERS", issued Sep. 29, 1987 to R. Harvey.
A typical FEL has a dispersion curve as illustrated in FIG. 1. For a given waveguide mode and voltage applied to electrostatically accelerate the electron beam, a conventional cavity resonator will produce gain at two discrete frequencies such as designated at 10 and 12 in the drawing. The higher frequency 10, which is the desired microwave or optical emission frequency of the laser and is approximately 30 GHz in the illustrated example, corresponds to the forward propagation mode, whereas the lower, undesired frequency 12 is approximately 10 GHz and corresponds to the backward propagation mode.
Conventional cavity resonators can be designed to have a high quality factor, Q, for the high frequency mode. However, if the low frequency mode is near the waveguide cutoff frequency of the waveguide member, the Q for the low frequency mode will also be high, and the two modes can compete with each other. When this happens, the gain of the desired high frequency mode is degraded.
Suppression of undesired frequency modes in waveguide structures such as cavity resonators has been accomplished in the past by providing tuned structures inside the waveguides which pass signals only in selected frequency ranges. The theory and design of conventional waveguide filters is described in detail in a textbook entitled "MICROWAVE TRANSMISSION CIRCUITS", edited by G. Ragan, McGraw-Hill 1948, pp. 540-716. However, these filters are often inappropriate for use in a cavity resonator of an FEL because they have unacceptably narrow bands of operation.